1. Field of the Invention
The present invention relates generally to processes for producing thin films with low impurity contents on a substrate by atomic layer deposition. In some embodiments, the films produced by the atomic layer deposition (ALD) processes disclosed herein can be used in metal gate and metal electrode applications in metal oxide semiconductor field effect transistors (MOSFETs) or as barrier layers in interconnect structures.
2. Description of the Related Art
Atomic layer deposition (ALD) is a self-limiting process, whereby alternated pulses of reaction precursors saturate a substrate surface and leave no more than one monolayer of material per pulse. The deposition conditions and precursors are selected to ensure self-saturating reactions, such that an adsorbed layer in one pulse leaves a surface termination that is non-reactive with the gas phase reactants of the same pulse. A subsequent pulse of different reactants reacts with the previous termination to enable continued deposition. Thus, each cycle of alternated pulses leaves no more than about one molecular layer of the desired material. The principles of ALD type processes have been presented by T. Suntola, e.g. in the Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics, Chapter 14, Atomic Layer Epitaxy, pp. 601-663, Elsevier Science B.V. 1994, the disclosure of which is incorporated herein by reference.
In a typical ALD process for depositing thin films, one deposition cycle comprises exposing the substrate to a first precursor, removing unreacted first reactant and reaction byproducts from the reaction chamber, exposing the substrate to a second precursor, followed by a second removal step. Typically, halide precursors, such as TiCl4 and HfCl4, are used as precursors in ALD deposition because those precursors are inexpensive and relatively stable, but at the same time reactive towards different types of surface groups. H2O and NH3 are widely used for oxide and nitride deposition, respectively, as second precursors.
ALD processes typically produce thin films that have lower impurity content at the same deposition temperature than chemical vapor deposition (CVD) processes. Despite the lower impurity levels in ALD films, the impurity content in ALD films can still be a problem. There are several possible reasons for the presence of impurities in thin films deposited by ALD. In some cases, the semiconductor process flow necessarily limits the maximum deposition temperature such that that some residues are left in the film. ALD films deposited from chloride or other halide-containing precursors (e.g., WF6) at relatively low temperatures can comprise relatively high levels of halide residues. Halide impurities are present mainly at the interfaces, which can also lead to problems. In some cases, like low temperature deposition of transition metal nitrides and transition metal carbides from halide containing precursors the, impurity contents can be above the acceptable limit for some integrated circuit (IC) applications. In another example, in some applications amorphous films are needed, which limits the growth temperature.
Another disadvantage of the residues remaining in the film as it is deposited is that they may block the growth and result in a lower growth rate. For example, a high growth temperature may be chosen because the films are impure at low temperatures. However, the number of reactive active sites, such as —OH or —NHx groups, is higher at low temperatures As a result, the growth rate is substantially lowered by impurities.
U.S. Patent Application No. 2004/0208994 to Harkonen et al. describes a method for ALD deposition of carbon-containing transition metal films. As an example, Harkonen et al. deposited carbon containing titanium films (example 1B) at a deposition temperature of about 500° C. using TiCl4 and trimethylaluminum (TMA) as precursors. The disadvantage of this process is that it needs a substantially high deposition temperature in order to achieve low impurity contents, chlorine in their case. Furthermore, it is widely known in art that TMA will decompose when used at such high temperatures. By decomposing TMA, the uniqueness of ALD, i.e., saturated and surface controlled reactions, which leads to superb conformality and uniformity of ultra-thin films over the large-area substrates, may be lost. If the same carbon containing titanium film process is performed at temperatures below the decomposition temperature of TMA, for example at 350° C., the chlorine content of the film will be undesirably high.
Accordingly, there is a need in the art for a low temperature ALD method for producing metal-containing films from halide (e.g., chlorine) containing metal precursors at low temperatures and with low halogen impurity levels in the films.